If you want to decode the encrypted sniffed data then you have to configure your sniffing set-up appropriately. Normally this requires observing the pairing process so that the sniffer can also compute the link key (and therefore encryption key). If using Bluetooth v2.0 and earlier you may be able to enter the PIN code you are going to use in the sniffing tool prior to performing pairing. If using Bluetooth v2.1 and later you will need to configure one of the Bluetooth devices to enter "SSP debug mode".

For the latter (Bluetooth v2.1 and later) case, we have a patch that adds the ability to enable/disable SSP debug mode using the hciconfig tool - I'll push it out to the mailing list as soon as possible, as it appears it would be useful for others.